It is known that pharmaceutical preparations can be produced as uniform granules from a mixture of powdered ingredients, which powdered ingredients are of different particle size, bulk density, and specific weight and are present in differing proportions. The powdered ingredients are placed into a closed container having a perforated base through which heated air is driven by a fan from below the base. This air flow is so regulated in relation to the container and the powdered mixture therein that the powder particles are upheld or suspended in what is known as a fluidized bed. A thorough mixing of the individual powder particles takes place in the fluidized bed. Subsequently, or simultaneously, there is sprayed into the fluidized bed, by means of a nozzle a liquid which usually contains a binder. Powder components are dissolved by the liquid, so that subsequently they can act as binding agents. When drops of liquid are combined with powder particles, the particles are able to adhere to other powder particles by means of liquid bonds. The liquid evaporates as a consequence of the high air through-put, and bonds of solid material are formed between the powder particles, by crystallization of the substances dissolved within the liquid or by hardening of the binders.
In contrast to other known granulating techniques, wherein the granules are formed by the pulverization of relatively large agglomerates, the granulate formation in the above-described fluidized bed spray granulating process occurs by a continuous enlargement of the original powder particles.
In the above-described fluidized bed spray granulating process, the more moist the mass of powder materials is maintained during the spraying phase, that is, the faster the granulating liquid is sprayed in, the more rapidly the size of the individual granules (among other things) is increased. For economy reasons, it is desirable to reduce the spray time by spraying the liquid more quickly. However, if the spray time is reduced too greatly, agglomerates the size of snowballs are formed. These agglomerates only dry on their outer surfaces during the subsequent drying phase, whereas in their interior they remain moist, so that the mass as a whole is non-uniformly dried. If the spraying time is reduced even further, the entire mass forms a damp cohesive cake on the perforated base, into which the airflow penetrates through cracks and channels. However, this damp cohesive cake can no longer be lifted up by the airflow alone and no longer forms a fluidized bed. Therefore, the speed in the fluidized bed spray granulating process, is correspondingly limited in each case according to the quality of the granulate.